1. Field of the Invention
The present invention relates to a hydrodynamic machine, in particular a hydrodynamic retarder, which is incorporated in an external working medium circuit and a method for minimising the drag power of such a hydrodynamic machine.
2. Description of the Related Art
Hydrodynamic machines having an external working medium circuit differ from hydrodynamic machines without an external circuit in that the working medium does not remain permanently inside the hydrodynamic machine, for example, either in the working chamber or a store chamber provided in the hydrodynamic machine—also called retarder chamber—but is passed from an external working medium circuit in which a cooler is usually provided for the working medium, into the hydrodynamic machine or the working chamber thereof and is then removed from the working chamber or the hydrodynamic machine into the external working medium circuit. Accordingly the hydrodynamic machine has at least one working medium inlet and one working medium outlet. A pressure regulating valve, for example, is provided in the working medium outlet or behind this in the direction of flow, by which means the static pressure in the working chamber and therefore the filling level of the working chamber is adjusted.
The removal of working medium from the working chamber and when switching off the hydrodynamic machine, the emptying of the working chamber is accomplished by means of so-called outlet holes. These outlet holes are provided distributed over the circumference of the hydrodynamic machine (when viewed in the direction of rotation) in the blades or in the intermediate spaces between the blades, in the case of a retarder, usually the stator, and open on the one hand in the working chamber and on the other hand in an annular channel extending in the circumferential direction (direction of rotation) of the hydrodynamic machine. The annular channel is used for collecting the working medium, that is the flow of working medium from the individual outlet holes is combined in the annular channel. From the annular channel the working medium is then passed via the said working medium outlet, which is disposed in relation to the direction of flow in series with the outlet holes, into the external working medium circuit.
A disadvantage of the known designs is that the hydrodynamic machine, particularly when this is designed as a retarder, and the outlet holes in the stator which are provided in a not-revolving blade wheel, do not empty completely since particularly in a partially filled state of the working chamber, that is, when the maximum possible amount of working medium is no longer present in the working chamber, the pressure losses in the working medium flow through the outlet holes prevent this. In such a partially filled state of the working chamber, the pressure in the working chamber driving the emptying is lower than that in the fully filled state.
DE 10 2007 029 018 describes such a hydrodynamic machine having a primary wheel and a secondary wheel forming a working chamber which can be filled via an external working medium circuit in order to accordingly adjust the power transmission of the hydrodynamic machine. Further provided is an annular channel which is connected in a working-medium conducting manner with the working chamber via outlet holes in the primary wheel and the secondary wheel. As a result of emptying problems, in addition to the outlet holes, a working medium outlet is provided which opens in the working chamber and is disposed parallel to the annular channel. The features of this document are summarized as follows: A hydrodynamic machine, in particular a retarder, having an external working medium circuit, comprising: a bladed primary wheel and a bladed secondary wheel which together form a toroidal working chamber which can be filled with working medium from the external working medium circuit, which is fed back into the external working medium circuit; an annular channel disposed outside the working chamber, which is connected in a working-medium conducting manner to the working chamber via outlet holes in the primary wheel or secondary wheel, and is in flow-conducting communication with the external working medium circuit in order to collect working medium leaving the working chamber through the outlet holes and supply this to the external working medium circuit.
Experiments have now shown that hydrodynamic machines as described by the above document reduce the drag power (transmitted power as a result of a residual quantity of working medium in the working chamber when the hydrodynamic machine is deactivated). Accordingly, the drag power is not optimal at each rotational speed so that at certain rotational speeds the hydrodynamic machine transmits undesired power which can lead to an undesired power loss in the drive train in which the hydrodynamic machine is used.
It is the object of the present invention to provide, and what is needed in the art is, a hydrodynamic machine in which the extensive emptying of the working chamber is improved compared with the prior art and a method for minimizing the drag power at all rotational speeds.